1. Field of the Invention
The present invention relates generally to magnetic field sensors, and more specifically, to a converter circuit for providing a digital output from a semiconductor magnetic field sensor.
2. Background of the Invention
Hall effect sensors and other semiconductor magnetic field sensors are widely used in applications in which it is desirable to provide a measurement of DC magnetic fields and relatively low frequency AC magnetic fields that are not otherwise easily sensed with coils or other antennas. Such applications include position and motion sensors for both linear and rotational motion, power supply and motor control applications in which the transformer or motor fields are detected, audio speaker applications in which the strength of the speaker's signal-induced field is detected, and lighting controllers for high-frequency energized lamps, such as sodium lamps.
Semiconductor magnetic field sensors typically vary widely in their output voltage vs. field strength sensor function, both with temperature and from device to device. Background magnetic fields may also contribute to measurement error, depending on the proximity of magnetic field sources and/or the strength of the field being measured with respect to normal background magnetic field strength.
Typical circuits for measuring the output of a semiconductor magnetic field sensor include circuits that connect a Hall effect or other sensor in the feedback path of an operational amplifier circuit, which is then provided to an analog-to-digital converter circuit or an analog comparator that detects the sensor output. Calibration in offset and gain provide for compensation for variation between devices, and temperature compensation may also be included in the amplifier circuit.
More recently, sensor implementations have been proposed that impose an AC magnetic field on the sensor, which can then be detected at the sensor output and used to calibrate the sensor circuit. One or more current loops is implemented around the sensor and stimulated with a signal having known characteristics, inducing an AC magnetic field at the sensor, which causes an AC voltage of the same frequency profile in the sensor output voltage. The sensor output can then be detected and the gain of the sensor circuit adjusted to calibrate the sensor. However, such a circuit has significant extra complexity, since a calibration source and detector are required, and the calibration must be either periodic, or out of the band of the desired detection range of the sensor circuit, which will typically require additional filtering circuitry.
Therefore, it would be desirable to provide a semiconductor magnetic field sensor measurement circuit that is calibrated via an induced magnetic field, without requiring much additional complexity in the measurement and calibration circuits.